U.S. Pat. No. 4,629,676 (Hayakawa et al) and European Patent Publication No. 0174634B1 disclose an image forming method comprising the steps of imagewise exposing to light a light-sensitive material, and heating the light-sensitive material. The light-sensitive material comprises a support and a light-sensitive layer containing silver halide, a reducing agent and a polymerizable compound. The light-sensitive material is heated to develop the silver halide and to harden the polymerizable compound.
The support of the light-sensitive material can be made of various substances, such as a paper, a synthetic paper, a plastic film and a metal plate. A paper laminated with a synthetic resin (e.g., polyethylene, polypropylene, polystyrene) is also available. The paper or the plastic film may be laminated with a metal. Further, a metal may be evaporated the paper or the plastic film.
The above-mentioned image forming method is available for forming a lithographic plate. U.S. Pat. No. 5,122,443 (Takeda) and European Patent Publication No. 0426192B1 disclose a method of forming the lithographic plate.
The support of the lithographic plate is preferably made of an aluminum plate, a polyethylene terephthalate film, a paper or a synthetic paper. The aluminum plate is particularly preferred.
The aluminum support has been used in not only the above-mentioned method using silver halide but also conventional methods of forming lithographic plate using a photopolymer. The aluminum support usually has an anodic oxide layer.
The surface of the aluminum support may be treated to form a rough surface (graining treatment) or a hydrophilic surface (Japanese Patent Publication No. 47(1972)-5125 and U.S. Pat. Nos. 3,834,498 and 4,087,341). An intervening (undercoating) layer of a hydrophilic compound may be provided on the aluminum support to decrease stains in printed matters (U.S. Pat. Nos. 2,098,627 and 3,860,426 and Japanese Patent Provisional Publications No. 60(1985)-149491 and 4(1992)-282637). The stains are caused by contamination of a light-sensitive layer within non-image area. The amounts of impurities in the aluminum support are preferably reduced to prevent the printed matters from ink stains, as is described in Japanese Patent Provisional Publications No. 58(1983)-11759, No. 58(1983)-221254, No. 62(1987)-146694 and No. 62(1987)-148295. Examples of the impurities include silicon, copper and iron. The impurities attach ink to non-image area, which causes the ink stains.